2.1. Cytokines in Septic Shock
Septic shock is a multifaceted pathological condition characterized most prominently by deleterious hemodynamic changes and coagulopathy leading to multiple organ failure and often to death. The altered physiological mechanisms underlying the septic shock syndrome, and the cellular means by which these changes are induced and controlled, are not yet known in precise detail. In broad outline, however, a consensus view of events culminating in septic shock has emerged over the last several years. In particular, it is now generally accepted that septic shock reflects the individual, combined and concerted effects of a large number of endogenous, host-derived mediator molecules. These mediators are produced in response to initiating stimuli that indicate the host has been invaded, and the class of peptide mediators that were generally first recognized as white blood cell products have come to be known as cytokines. As mediators of toxic effects and pathological alterations in host homeostasis, these endogenous factors represent potentially attractive therapeutic targets, and septic shock remains a potentially lethal cytokine-mediated clinical complication against which there is no generally effective therapeutic approach.
Although traditionally termed "septic" shock, infection by a variety of microorganisms including not only bacteria but also viruses, fungi, and parasites can induce septic shock. In fact, the shock syndrome is more properly associated with the host's response to invasion rather than just infection, as cancer and trauma, for instance, can also serve as initiators. In the case of infection by gram-negative bacteria, one of the best studied examples, it is believed that the appearance of bacterial endotoxins such as lipopolysaccharide (LPS) in the host bloodstream leads to the endogenous production of a variety of host factors that directly and indirectly mediate the toxicity of LPS, which itself is relatively innocuous for most cells. These host-derived mediators include many now well-recognized inflammatory cytokines and classical endocrine hormones in addition to a number of other endogenous factors such as leukotrienes and platelet activating factor. It is generally acknowledged, however, that the full cast of participants and each of their interrelated roles in the host response remains incompletely appreciated.
In general, those mediators that appear earlier in an invaded host are thought to trigger the release of later appearing factors. Also, many endogenous mediators not only exert direct effector functions at their target tissues, but also prime local and remote tissues for subsequent responses to other mediators. This interacting network of host factors has been termed the "cytokine cascade." This term is meant to indicate the rapid extension and amplification of the host response in such a way that only one or a few initiating stimuli trigger the eventual release and participation of scores of host mediators. Although a number of features of the host response are thought to assist in fighting off invasion, an overly robust or poorly modulated endogenous response can rapidly accelerate to rapidly produce such profound alterations in host homeostasis at the cellular, tissue, and systemic levels that death may ensue within hours.
Among the interacting factors that together comprise the cytokine cascade, the cytokine known as tumor necrosis factor-alpha (TNF.alpha.) is the most important identified to date. TNF.alpha. is the first cytokine to appear in the circulation after LPS challenge. The hemodynamic and metabolic alterations that result from the experimental administration of TNF.alpha. closely resemble those that have been observed in endotoxemia and septic shock. In animal models, TNF.alpha. is the only host factor which itself can initiate a lethal syndrome that mimics septic shock in detail. In this respect, TNF.alpha. can be considered a sufficient mediator of septic shock. Functionally neutralizing TNF.alpha. antagonists such as anti-TNF.alpha. antibodies are protective in otherwise lethal bacterial infections, and in this respect TNF.alpha. can be considered a necessary mediator of septic shock.
Other cytokines participate in the host response to LPS but appear later in the circulation. However, no other cytokine has been shown to be both necessary and sufficient to mediate septic shock. For example, certain interleukins (IL-1, IL-6 and IL-8) which appear in serum more than 2 hours after LPS challenge, and interferon .gamma. (IFN-.gamma.) which appears after 6 hours, are thought to play a significant role in septic shock, and can be shown to contribute to lethality in certain disease models or under experimental conditions of endotoxemia. Antagonism of the effects of specific interleukins and interferons has been shown to confer a significant protective effect under certain conditions. Nevertheless, none of these other factors can itself induce a full-blown septic shock-like effect in otherwise healthy individuals, and none of these other cytokines appears to play as central and critical role in septic shock as TNF.alpha..
In view of the foregoing, TNF.alpha. stands as an ideal target for the treatment of septic shock. Unfortunately, temporal characteristics of the endogenous TNF.alpha. response suggest a significant practical limitation for this potential therapy. TNF.alpha., one of the earliest elicited mediators to appear in acute disease, rapidly peaks after bolus endotoxin challenge (30-90 minutes), and diminishes just as promptly. It is presumed that most of the damaging effects of TNF.alpha. in septic shock are completed during this early period, hence TNF.alpha. antagonists such as anti-TNF.alpha. antibodies would ideally be present at this time. Since this therapeutic window is apparently so short and occurs so early, the timely delivery of anti-TNF.alpha. based therapeutics may be very difficult to achieve clinically.
Therefore, in order utilize cytokines as targets for the treatment of septic shock and other cytokine-mediated toxic reactions, there exists a desperate need to discover additional targets that are both necessary components of the cytokine cascade and occur at a time during the endogenous response that is accessible for therapeutic antagonism in the course of clinical treatment.